vCenter Operations Manager is an application that consists of two virtual machines in a vApp-container, the UI-VM and the Analytics-VM. (There is also an installable version of vCops that you can install on Windows or Linux, but this article is only for the Appliance-based installation). When one of these two virtual machines starts to run out of disk space then it's very simple to increase the size of the data disk. The reason is that VMware has used Logical Volume Manager (LVM) which allows you to increase the size of a volume simply by adding a new empty hard disk. It then automatically adds that empty disk to the configured data-volume.
This is described in this VMware knowledge base article: KB2016022
In the example below I have used these three commands to show the disk space of the Analytics-VM before adding a hard disk:
pvdisplay - to show the physical disks used for the data_vg volume group
lvdisplay - to show the configuration of the logical volume
df -h - to show the available disk space of the mounted data volume
As you can see in the image above there is a 200GB volume with only one physical disk (/dev/sdb). To extend the disk size you need to add a disk and then just restart the virtual machine. This works for both the Analytics-VM and the UI-VM but since the Analytics-VM is the one storing the data I have that one in this instruction. From the vSphere Web Client open the virtual machine settings and add a new hard disk with enough additional disk space, such as the 60GB I have added in this example. (VMware suggests to configure the disk as an Eager Zeroed Thick disk.)
After the disk was added and the virtual machine is started you can verify that the new disk has been added to the volume group. In this example disk /dev/sdc has been added to the data_vg volume which is now 260GB in size.
It is possible to remove a disk from the volume group but you can only do that when the disk has not been used to store data. So for example right after adding the disk if you made a mistake it might be possible, but after days or weeks of operation the disk is most likely used and can no longer be removed. How to remove a disk is described in this VMware knowledge base article: KB2046813